1. Field of the Invention
The present invention relates to a combustion state detecting device that detects a combustion state of an internal combustion engine by detection of a change in the quantity of ions which are produced at the time of burning in the internal combustion engine, and more particularly to a combustion state detecting device for an internal combustion engine which is capable of accurately conducting knocking detection or misfire detection.
2. Description of the Related Art
In general, in an internal combustion engine driven by a plurality of cylinders, the fuel-air mixture consisting of air and fuel introduced into the combustion chambers of the respective cylinders is compressed by moving up pistons, electric sparks are generated by applying an ignition high voltage to ignition plugs located in the respective combustion chambers, and an explosion force developed at the time of burning the fuel-air mixture is converted into a piston push-down force, to thereby extract the piston push-down force as an rotating output of the internal combustion engine.
There has been known that since molecules within the combustion chambers are ionized when the fuel-air mixture has been burned within the combustion chambers, ions having electric charges flow between the ignition plugs as an ion current upon application of a bias voltage to ion current detection electrodes (as usual, ignition plug electrodes are used) located within the combustion chambers.
Also, there has been known that the combustion state of the internal combustion engine can be detected by detection of a state in which the ion current occurs because the ion current is sensitively varied according to the combustion state within the combustion chambers.
FIG. 8 is a circuit block diagram showing one example of a conventional combustion state detecting device for an internal combustion engine.
In the figure, an anode of a battery 1 mounted on a vehicle is connected to one end of a primary winding 2a of an ignition coil 2 whereas the other end of the primary winding 2a is connected to the ground through a power transistor 3 having an emitter thereof grounded for interrupting the supply of a primary current.
A secondary winding 2b of the ignition coil 2 constitutes a transformer in corporation with the primary winding 2a, and a high-voltage side of the secondary winding 2b is connected to one end of an ignition plug 4 corresponding to each cylinder (not shown) to output a high voltage of negative polarity at the time of controlling ignition.
The ignition plug 4 made up of counter electrodes is applied with an ignition high voltage to discharge and fire the fuel-air mixture within each of the cylinders.
The ignition coil 2 and the ignition plug 4 are disposed in parallel for each of the cylinders, however, in this example, only one pair of ignition coil 2 and ignition plug 4 are representatively shown.
A bias circuit 5 includes a capacitor 5a connected to a low-voltage side of the secondary winding 2b, a bias voltage limit Zener diode 5b connected in parallel with the capacitor 5a, and a diode 5c disposed between the capacitor 5a and the ground. A current-voltage convertor circuit 6 includes a resistor 6a connected in parallel with the diode 5c.
A series circuit consisting of the capacitor 5a and the diode 5c and the Zener diode 5b connected in parallel with the capacitor 5a are disposed between the low-voltage side of the secondary winding 2b and the ground, to thus constitute a charging path for charging the capacitor 5a with the bias voltage at the time of generating the ignition current.
During the off state of the power transistor 3 (at the time of interrupting the supply of a current to the primary winding 2a), the capacitor 5a is charged with the secondary current that flows through the ignition plug 4 discharged by a high voltage outputted from the secondary winding 2b. The charge voltage is limited to a given bias voltage (for example, about several hundreds V) by the Zener diode 5c and functions as the ion current detection bias means, that is, a power supply.
The resistor 6a within the current-voltage convertor circuit 6 converts an ion current allowed to flow by the bias voltage into a voltage to output the voltage thus converted to a knock signal generator circuit 7 and a delay circuit 8 as an ion current detection signal. The knock signal generator circuit 7 is made up of a filter circuit 7a and a comparator circuit 7b. The filter circuit 7a extracts a high-frequency vibration component contained in an ion current detection waveform at the time of generating knocking. The comparator circuit 7b compares the output of the filter circuit 7a with a given reference value Vc and converts a comparison result into a rectangular wave.
The delay circuit 8 includes an operational amplifier 8a, a resistor 8b connected between a positive power supply terminal V.sub.B and the ground, and a capacitor 8c. The non-inverse input terminal of the operational amplifier 8a is connected to the output side of the current-voltage convertor circuit 6, the inverse input terminal thereof is connected to a negative power supply terminal having a given reference value Va, and the output terminal thereof is connected to a node of the resistor 8b and the capacitor 8c.
A comparator circuit 9 includes an operational amplifier 9a and a resistor 9b. The non-inverse input terminal of the operational amplifier 9a is connected to the output side of the delay circuit 8, the inverse input terminal thereof is connected to a negative power supply terminal having a given reference value Vb, and the output terminal thereof is connected to the positive power supply V.sub.B through the resistor 9b and also connected to the base of the transistor 10. The emitter of the transistor 10 is grounded, and the collector thereof is connected to the positive power supply terminal V.sub.B through a resistor 11 and also connected to the base of a transistor 12.
The emitter of the transistor 12 is grounded, and the collector thereof is connected to the output side of the comparator circuit 7b, connected to the base of a transistor 14 and also connected to the positive power supply terminal V.sub.B through a resistor 13. The emitter of the transistor 14 is grounded, and the collector thereof is connected to an ECU (electronic control unit) 15. Structural elements 5 to 14 constitute a fuel state detector circuit 20.
The ECU 15 made up of a microcomputer judges a combustion state of the internal combustion engine on the basis of the ion current detection signal, and if the ECU 15 detects the deterioration of the combustion state, it appropriately conducts adaptive control so as not to cause any inconvenience.
Also, the ECU 15 arithmetically operates an ignition timing, etc., on the basis of drive conditions obtained from a variety of sensors (not shown), and outputs not only an ignition signal to the power transistor 3 but also a fuel injection signal to an injector (not shown) for each cylinder, and drive signals to a variety of actuators (a throttle valve, an ISC valve, etc.).
Subsequently, the operation of the conventional combustion state detecting device for an internal combustion engine shown in FIG. 8 will be described with reference to FIGS. 9A to 9F. The left side of FIGS. 9A to 9F shows signal waveforms appearing in the respective circuit portions when the internal combustion engine is in a low-revolution state, whereas the right side of FIGS. 9A to 9F shows waveforms when it is in a high-revolution state.
In general, the ECU 15 arithmetically operates the ignition timing, etc., in accordance with the drive conditions, and supplies an ignition signal P shown in FIG. 9A to the base of the power transistor 3 at a desired control timing to control the on/off operation of the power transistor 3.
As a result, the power transistor 3 interrupts the supply of the primary current flowing in the primary winding 2a of the ignition coil 2 to boost the primary voltage, and also develops an ignition high voltage (for example, several tens kV) at the high-voltage side of the secondary winding 2b.
The secondary voltage is applied to the ignition plug 4 for each of the cylinders and allowed to generate a discharge spark within the combustion chamber of an ignition control cylinder to burned he fuel-air mixture. In this situation, if the combustion state is normal, a required quantity of ions are generated in the periphery of the ignition plug 4 and within the combustion chamber.
Then, as described above, when the power transistor 3 is turned on in response to the ignition signal P, the supply of the current to the primary winding 2a starts, to thereby develop the voltage of the positive polarity at the high-voltage side of the secondary winding 2b.
Sequentially, at the time of interrupting the primary current, if the ignition high voltage is developed at the high-voltage side of the secondary winding 2b to make the ignition plug 4 discharge, the secondary current charges the capacitor 5a up to a predetermined voltage.
Also, since ions are generated by the discharge of the ignition plug 4, the ion current i flows in a direction indicated by a broken-line arrow in FIG. 8, as a result of which an ion current detection signal S1 on which a noise component or a high-frequency component caused by knocking is superimposed is obtained as shown in FIG. 9B.
The ion current detection signal S1 is supplied to the delay circuit 8 where it is compared with a predetermined reference value Va so as to be outputted at the output side thereof as a signal S2 shown in FIG. 9c. The signal S2 is compared with a given reference value Vb by the succeeding comparator circuit 9 so that a pulse signal S3 shown in FIG. 9D which is delayed a predetermined amount from the rising of the ion current detection signal S1 is outputted at the output side of the comparator circuit 9.
The pulse signal S3 is supplied to the transistor 10 as a switching signal so that the transistor 10 turns on and the transistor 12 turns off when the pulse signal S3 is high in level whereas the transistor 10 turns off and the transistor 12 turns on when the pulse signal S3 is low in level.
On the other hand, the ion current detection signal S1 from the current-voltage convertor circuit 6 is supplied to the filter circuit 7a in the knock signal generator circuit 7, where a frequency band substantially corresponding to the knock of the internal combustion engine is extracted from the ion current detection signal S1, and a signal S4 shown in FIG. 9E is outputted at the output side of the filter circuit 7a.
The signal S4 is supplied to the comparator circuit 7b at a succeeding stage where the signal S4 is compared with a given reference value Vc, and its comparison result is outputted, as a pulse signal S5 substantially corresponding to the occurrence of knock as shown in FIG. 9F, to the output terminal OUT of the combustion state detecting device 20 through the final transistor 14 the on/off operation of which is controlled in response to the pulse signal S3 that is a switching signal from the comparator circuit 9.
In other words, since the transistor 10 turns on, and the transistor 12 is off when the pulse signal S3 from the comparator circuit 9 is high in level, the transistor 14 to which the output of the comparator circuit 7b is supplied as it is turns on when the output of the comparator circuit 7b is high in level (when the level of the signal S4 is larger than the reference value Vc) but turns off when it is low in level (when the level of the signal S4 is smaller than the reference value Vc), with the result that the pulse signal S5 shown in FIG. 9F is obtained at the output terminal OUT of the combustion state detecting device 20 as a signal corresponding to the occurrence of knock.
The conventional combustion state detecting device for the internal combustion engine thus structured suffers from problems stated below.
That is, the high-frequency vibration component of knocking occurs from the peak of the ion current detection signal waveform, and there is the possibility that noise components analogous to the high-frequency component caused by knocking are contained in an entire range of waveforms due to the distortion of the waveform caused by the fluctuation of the combustion state, the superimposing of extraneous noises, noises caused by ignition operation, or the like. For that reason, the conventional device removes the ignition noise components mainly immediately after discharging operation has been completed, by the provision of the delay circuit 8, the comparator circuit 9 and the transistors 10, 12 to make a given period after the ion current detection signal has occurred in a non-detection period.
On the other hand, the ion current detection signal varies in waveform according to the drive conditions of an engine, and also differs in time width depending on the conditions. As a result, a detection period gets prolonged more than a required detection period particularly in a low-revolution condition where the time width is wide, thereby leading to such problems that the S/N ratio is deteriorated by taking more noise components, and the accuracy in detection of the combustion state of the internal combustion engine, in particular, the accuracy in knock detection is deteriorated.